Anti-aliasing and Anisotropic filtering demystified

Anisotropic filtering

Another method used to enhance 3D images is called Anisotropic filtering, this method improves the overall sharpness and clarity in a 3D scene. To explain how antisotropic filtering works we need to look at how a 3D scene is compiled first. As you might know texturing technologie is used to give objects in a scene the correct look. A texture is nothing more than a 2D picture that is applied to a 3D object. For instance a wall in a 3D scene will get a "brick" texture to make it look real.

A texture is a certain size (256 x 256 pixels) and will never have the exact size needed in the 3D scene in question. A wak can be far away in a scene and in reality is only a few pixels high. To apply the texture to the object so it looks real we need filtering.

When the first 3D games were released (Wolfenstein 3D, Doom, etc) developers used a method called point filtering. This is simply copying the pixel of the texture (texel) that is closest to the position of the 3D object onto the actual object. Bilinear filtering takes things a bit further, and in this method 4 texels are averaged out to decide which color the pixels should have, the end result of this is alreasy a lot more lifelike. In modern games the standard is Trilinear filtering, this method uses the different versions of the textures supplied by the game developers (i.e 64x64, 128x128 256x256, and so on). When using trilinear filtering a texture will first be rendered through bilinear filtering, however two diferent sized textures are rendered at the same time. The two results are combined and apllied in the end result, which looks more lifelike and clearer than the same texture rendered through bilinear filtering.

The techniques can still cause problems though, for instance when the object that the texture is applied to is at a large angel compared to the screen. This can cause a blurry image as the textures are smeared out, a good example of a situation like this is the floors that you walk on in 3D first person shooters.

The floor in Prey, with and without anisotropic filtering. The left image is quite blurry, where the right image is quite clear.

Anisotropische filtering will enhance the overall image, even when textures have to be applied to angles. This filtering techique will take the actual angle in account, and apply the samples accordingly. As with FSAA the use of AF will affect the performance of your graphics card, and you can set the amount of samples you wish to use. Obviously 8x AF will deliver a better end result than 2x AF, but the performance will be lower.